Pressure is growing on firms to improve customer service after complaints during the first half of the year more than doubled to 22,671 – up from 10,598 in the last six months of 2013.

June was also a record for the highest number of complaints ever received in one month, with 4,124 unhappy consumers making their frustration known.

Billing issues are the main problem, accounting for 84 per cent of the complaints made between January and June, according to the Energy Ombudsman.

Many relate to late or inaccurate bills, as energy firms increasingly rely on estimates instead of door-to-door meter readings.

Chief Energy Ombudsman Lewis Shand Smith said: 'The spike in complaints is in part a result of the rising cost of living, but also as a result of consumers becoming more aware of their rights and feeling more empowered to act and fight for a fair deal.

Rankings: Npower received considerably more complaints than any other energy supplier, according to Citizens Advice

'Addressing these concerns is crucial to restoring consumer confidence in the sector.'

Complaints about energy giants Npower and Scottish Power almost doubled in the first three months of this year as billing problems left thousands of customers with incorrect or missing bills, a similar survey revealed last week.

Npower remained ‘firmly at rock bottom’ in the complaints table, and Scottish Power fared little better in fifth place, after both introduced new billing systems.

Npower complaints rose from 306.8 for every 100,000 customers in the last quarter of last year to 592.4 between January and March – the equivalent of one complaint for every 168.8 customers, according to the survey from Citizens Advice.

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This is Money, Money Mail and the Financial Mail on Sunday receive a steady stream of correspondence from readers who have suffered poor customer service at the hands of the supplier.

Npower has been given just three months to sort out its billing problems or it will be forced to stop all telephone sales to new customers, the regulator warned last month.

A Department of Energy and Climate Change spokesman said: 'It is unacceptable that so many people have needed to complain about their energy suppliers.

'Energy companies need to realise that people will not tolerate poor service and are switching suppliers in unprecedented numbers, particularly to small suppliers whose numbers have nearly trebled since 2010.'

A spokesman for Energy UK said: 'The energy industry works hard to provide the best service for its customers but in an industry serving 27million households sometimes things go wrong.

'However, no one wants to see complaints rise and each complaint is taken very seriously with companies working hard and investing in resources and new systems to resolve issues as quickly as possible. Most complaints are dealt with by the end of the next working day with no more than a phone call.

'The industry will be consulting stakeholders this summer about the complaints it cannot fix in 48 hours.